I don't think they told "a joke", nor intended to. This was a humorous exchange as well as a commentary on human nature. My day is better for having read it.
Bill Watterson is one of the greatest comic artists ever, and even he said (paraphrased) "I enjoy a funny conversation more than just one punchline".
Another great person, Empricorn said "let people enjoy what they like".
And to deliver this profound message of "people are inherently distrustful" they needed 20 panels.
They could have done this in one. Have each caveman holding out one hand to pass food and the other hand holding a club behind their backs.
Want to really illustrate the groundbreaking idea that people don't trust each other? Make a second panel with knights replacing the cavemen and swords replacing clubs, then a third panel swapping in businessmen holding pistols.
If humor wasn't the goal that's... fine, but being long-winded in a format based on brevity undermines the message. Using 20 panels guarantees that half of the people who bother to look at the comic won't finish it. Those that do will probably be bored or even resentful that their time was wasted, making them less receptive to the message.
I find it interesting that your takeaway was "people are inherently distrustful." While there is truth to that, my interpretation was that "progress can be slow, but it is progress nonetheless." In this case the "slow" of the message was communicated through the panel count.